The invention relates to the field of generation of electrical signal transitions with very short fall times in the picosecond and subpicosecond range. More specifically, the invention relates to the field of compression of the fall time of an input step function to generate an output step function which has a fall time measured in picoseconds or fractions of picoseconds.
The development of advanced high-speed digital devices and integrated circuits has been spurred by a number of applications, including fiber optic digital data transmission at gigahertz rates, high-throughput computing and wide-band signal processing. Military interest in high-speed digital electronics stems from the need in electronic warfare systems to rapidly acquire, digitize and process very large amounts of data. Associated with the needs for high-speed logic are commensurate demands for broad-band analog signal processing, interface and input/output electronics, including correlators, and broad-band adaptive filters, picosecond-resolution sample-and-hold gates, and multi-gigahertz rate D/A and A/D converters. Time-domain electronic instrumentation of comparable speed will be required for the development of these components.
The primitive circuit functions for ultrafast analog/digital interface systems include sampling gates for acquisition of repetitive waveforms in measurement systems, sample-and-hold circuits as preprocessors for A/D converters, comparators, D/A and A/D converters, and picosecond pulse generators serving as the strobes for these circuits. To build such circuitry, it is necessary to be able to achieve pulse widths, aperture times, settling times and loaded-gate propagation delays on the order of from 1 to 10 picoseconds.
To attain switching speeds in the range of from 1 to 10 picoseconds, where both device and interconnection parasitics become important, novel circuit topologies incorporating microwave design methodology must be developed. State-of-the-art digital electronics and broad-band/pulsed analog electronic currently available both exhibit switching speeds and rise times on the order of 50 picoseconds which is equivalent to a bandwidth of approximately 8 Ghz.
The ability to generate signals having very short fall times is very useful. Such signals may be differentiated to generate pulses having very narrow pulse widths, and these pulses may be used to gate diode sampling bridges used in certain signal processing equipment such as sampling oscilloscopes to do wide-bandwidth time-domain electronic measurements. Such short pulses are also useful in other waveform sampling devices and devices to do picosecond metrology as well as in the applications mentioned previously herein. In general, picosecond pulse generators are central components in highspeed analog applications and in high performance digital systems.
In the prior art, step recovery diodes have been used to generate 10 volt transitions of approximately 35 picosecond rise times for use in gating diode sampling bridges For purposes of discussion the term "rise time" will be considered equivalent to the term "fall time". Tunnel diodes have also been used to generate short pulses, and step functions or transitions. These pulses have been used as the test pulse in time domain reflectometers. These tunnel diodes generate transitions of 0.2 volts having 25 picosecond rise times.
Sampling oscilloscopes have used step recovery diodes for gating the diode sampling bridges therein since the early sixties. The performance of these oscilloscopes has not improved significantly since circa 1966-1968. In 1987, some new sampling oscilloscopes were introduced having 20 Ghz bandwidth. However, the earlier sampling oscilloscopes of 1970 vintage had a bandwidth of 18 Ghz, so the new oscilloscopes did not represent a major improvement over the older models.
Thus, a need has long existed for a device which can generate signals which have fall times in the range of from 1 to 10 picoseconds. These transitions can be used to generate very short pulses to trigger diode-sampling bridges and for many other purposes.
The elements to construct such an apparatus have been available in the prior art for many years. For example, nonlinear transmission lines have been available for approximately 30 years, it has long been known that fast electrical transients can be generated by nonlinear wave propagation on such nonlinear transmission lines through the formation of "shock waves". The meaning of this term will become clear from the following discussion. In an article entitled "Parametric Amplification along Nonlinear Transmission Lines" by Rolf Landauer published in the Journal of Applied Physics, Volume 31, Number 3, March 1960 at pages 479-483, distributed, nonlinear capacitances along a dispersionless transmission line were taught as a possible structure to provide parametric amplification. The structure he proposed was a ferroelectric crystal with thin metallic strips evaporated on its surfaces, serving as electrodes. He noted that such crystals at temperatures just above the transition at which the spontaneous polarization disappears are strongly nonlinear capacitances. Landauer also taught that the speed of a given part of a wave propagating down such a transmission line depends upon the voltage. This results in a distortion of the wave shape. This occurs because portions of each wave which are associated with the smallest values of differential capacitance will be the fastest. This causes these portions to tend to catch up with preceding slower portions and to simultaneously move away from slower portions of the wave that follow. Landauer taught that for a transmission line where dQ/dV is a monotonically decreasing function of voltage, a pulse having a Gaussian shape at the input will emerge with a distorted shape with the leading edge having a faster fall time than the signal which was input to the line. He also noted that eventually this pulse-shaped distortion will result in a wave which has a front with infinite slope. This process results in what are called "shock waves". In a later article published in the IBM Journal of October 1960, entitled "Shock Waves in Nonlinear Transmission Lines and Their Effect on Parametric Amplification" at pages 391-401, Landauer noted that the deformation process in a nonlinear transmission line is of some interest as a method of harmonic generation and as a method by which one end of a pulse can be sharpened at the expense of the other. This paper taught that as periodic signals are propagated along a transmission line with a nonlinearity in the distributed capacitance, the signal is deformed and electromagnetic shock waves formed. The paper teaches that these shock waves will form in a distance which is too short for any parametric amplification purpose. The paper concludes that parametric amplification cannot be achieved on transmission lines which are relatively dispersionless with nonlinear wave propagation properties that cause the formation of such shock waves. However, the paper does teach that wave shaping, harmonic generation and intermittent amplification accompanied by a signal compression are possible with such a system. Thus it has been known since at least 1960 that nonlinear transmission lines create changes in the input-pulse shape which can be used for wave shaping.
Other workers in the art have also investigated the generation of shock waves on nonlinear transmission lines. In a paper entitled "On the Theory of Shock Radio Waves in Nonlinear Lines", published in 1961 by Khokhlov in Radio Tekhnika i Elektronika, No. 6 at pp. 917-925, an analysis was given of the process of propagation of waves in weakly nonlinear and weakly absorbing media having no dispersion property. This paper referenced the work of Landauer and investigated propagation in a line where the nonlinear parameter was the distributed capacitance. The influence of attenuation on the formation and blurring of the shock wave front was analyzed, and the possibility of using such a line for a generator of harmonics and sawtooth voltages was presented. The influence of the series resistance in such a line was also analyzed. The paper concluded that the propagation of radio waves along a weakly absorbing line with nonlinear distributed capacitance causes the shape of the wave to be distorted and discontinuous. Eventually, the wave is gradually transformed into a sawtooth wave which is independent of the form of the input periodic voltage. Thus, the possibility of changing a wave shape by propagation of a signal down a nonlinear transmission line was again recognized in the prior art by a different worker than recognized this property the first time who noted the first workers findings and studied the subject in more detail.
Other workers in the art have proposed high frequency transmission lines which are periodically loaded with varactor diodes for the study of nonlinear wave propagation. "Nonlinear Wave Propagation Along Periodic Loaded Transmission Line" published by D. Jager and F. J. TeGude in Applied Physics Vol. 15, pp. 393-397 (1978) taught the transition capacitance of the PN junction of varactor diodes can be used to cause nonlinearity of wave propagation in a transmission line. This results from the fact that the capacitance of any particular section depends upon the voltage across the transmission line at that point. This paper focused not on wave shaping or compression of fall times but on harmonic frequency generation along a nonlinear transmission line. It concluded that second harmonic generation can be reached under certain circumstances.
In a later paper entitled "Characteristics of Traveling Waves Along the Nonlinear Transmission Lines for Monolithic Integrated Circuits: A Review" published in the Int'l Journal of Electronics, Vol. 58, No. 4 at pages 649-669 (1985), D. Jager taught a structure for a nonlinear transmission line using spatial periodicity. This spatial periodicity was implemented in the form of periodic loading of the line by Schottky diodes. This paper taught the use of in coplanar waveguides on layered semiconducting substrates. The paper proposes a structure for a Schottky microstrip line which is periodically loaded with diodes where approximately half the length of the line is consumed by diode active areas. FIG. 2 of this paper shows periodic, slow-wave propagation structures which are somewhat similar to the structure of the invention but which differ in several significant aspects. First there is no buried N.sup.+ layer of heavily doped semiconductor to reduce the diode series resistance. Second, there are apparently no ohmic contacts between the ground plane conductors and the doped semiconductor in the diode isolation island. Also, the pitch in center to center spacing appears to be higher than in the invention. If this is true, it would result in much more of the transmission line total area being consumed by diode junction area. This would result in much higher total capacitance, and in a characteristic impedance which is lower than the industry standard of 50 ohms. The paper concludes that the proposed slow-wave structures having the nonlinearity arising from the voltage dependence of the depletion layer width in the semiconducting substrate are the most promising devices for nonlinear wave applications. However, the paper also concludes that a central drawback limiting the usefulness of these structures exists in the attenuation of the wave during propagation. The paper notes that these losses must be minimized for any practical design. Generally speaking, this paper is focused upon the slowwave propagation characteristics of these lines and does not focus upon the possibilities of wave shaping or compression of fall times using such structures.
It has been known since at least 1966 that the speed at which the diode-sampling bridge in a sampling oscilloscope can be gated is the limiting factor on the bandwidth of such a device. However, the structure of the invention to solve this problem has not been previously disclosed in the prior art despite the fact that the various elements needed to make the structure and the basic theory of operation of the structure have long been known. Most, if not all, the structures and process steps needed to make the structure of the invention such as varactors, nonlinear transmission lines, heavily and lightly doped semiconductor layers, and ohmic contacts and epitaxial methods have been known in the prior art for quite some time. Thus, although the need for picosecond pulses has existed since the sixties and the tools to make a structure to fill this need have existed since the sixties and seventies, nothing in the prior art before this invention existed to fill the need. Basically this need is for a device which can compress the fall time of an input signal to somewhere in the range from 1 to 10 picoseconds and which has a characteristic impedance of approximately 50 ohms such that the device may be coupled to other transmission lines having the industry standard characteristic impedance of 50 ohms with good power transfer characteristics at the coupling. In some embodiments, it may be possible to compress the input fall time to the subpicosecond range.